There are varying challenges when choosing a bathtub. Those plans that you have with respect to your bathroom – what you have already and what you want with regards to the tub’s function - should factor in to your final decision.
I can think of several cross stitch patterns that could be enlarged and used for a pillow design. Cartoon characters with simple line art like those pictured would probably be best. Winne the Pooh, Hello Kitty, etc.